Description

Calculates any one parameter of the Poisson
distribution given values for the others.

Formula 26.4.21 of Abramowitz and Stegun, Handbook of
Mathematical Functions (1966) is used to reduce the computation
of the cumulative distribution function to that of computing a
chi-square, hence an incomplete gamma function.

Cumulative distribution function (P) is calculated directly.
Computation of other parameters involve a seach for a value that
produces the desired value of P. The search relies on the
monotinicity of P with the other parameter.

From DCDFLIB: Library of Fortran Routines for Cumulative Distribution
Functions, Inverses, and Other Parameters (February, 1994)
Barry W. Brown, James Lovato and Kathy Russell. The University of
Texas.